The capacitor for use in electronic devices such as cellular phone and personal computer is demanded to have a large capacitance with a small size. Among these capacitors, a tantalum capacitor and a niobium capacitor have a large capacitance for the size and also have good performance and therefore, these capacitors are being preferably used. In recent years, electronic devices are demanded to be driven at a low voltage and a high frequency with low noises and in order to cope with this, the solid electrolytic capacitor is also demanded to have higher capacitance, lower ESR (equivalent series resistance) and enhanced tan δ property.
The anode generally used in the capacitor using valve-acting metal is a sintered body of valve-acting metal powder. For example, a niobium powder is granulated by mixing the niobium powder with a liquid binder and then shaped by press-shaping and after implanting an anode lead thereto, the shaped article is sintered at a high temperature in a high vacuum to obtain an electrode called a sintered body. The inside of the sintered body takes a three-dimensional complicated form such that the powder particles are electrically and mechanically connected with each other. After a dielectric film is formed on the surface including the surfaces of inside voids of the sintered body, the sintered body is impregnated with a material working out to a counter electrode, thereby fabricating a capacitor. Microscopically, the capacitance of the produced capacitor greatly depends on the contacting state between the counter electrode material and the dielectric film layer as long as the dielectric film layer is uniformly attached to the surfaces inside and outside the sintered body.
In order to increase the capacitance of such a valve-acting metal capacitor, the mass of the sintered body must be increased or a sintered body increased in the surface area by finely pulverizing the valve-acting metal powder must be used.
The method of increasing the mass of the sintered body is disadvantageous in that the shape of the capacitor is inevitably increased and the demand for downsizing cannot be satisfied. On the other hand, the method of increasing the specific surface area by finely pulverizing valve-acting metal powder has a problem that the pore diameter of the valve-acting metal sintered body decreases or the number of closed pores increases at the sintering step to make it difficult to impregnate the sintered body with a cathode material in a later step.
Assuming that the capacitance realization ratio (also called an impregnation ratio of cathode material) is 100% when an aqueous phosphoric acid solution is used as the counter electrode material and complete contact state between the dielectric film layer and the cathode material is formed, for example, a capacitance realization ratio of 100% is difficult to attain in a case where an electrode material having large viscosity, especially a solid electrode material is used. In particular, when the average particle size of the valve-acting metal powder is small or when the sintered body produced from the valve-acting metal powder has a large shape, the difficulty increases. In an extreme case, only a capacitance realization ratio of less than 50% results. In such a case where only a low capacitance realization ratio can be attained, it is needless to say that a valve-acting metal capacitor having high capacitance cannot be produced and moreover, reduction of ESR, satisfactory tan δ property and sufficiently high humidity resistance cannot be obtained.
One of techniques considered to solve these defects in the valve-acting metal sintered body having high capacitance is a capacitor using, as the electrode, a sintered body where pores of the sintered body are enlarged to enhance the impregnation property of the counter electrode material having large viscosity, especially a cathode material which is a solid electrode material, and thereby enable the realization of high capacitance, low ESR, low tan δ value and long-term reliability.
JP-A-48-25859 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a porous electrode for capacitors, where a compound having an evaporation point lower than the sintering temperature is added to a metal powder such as tantalum or niobium and the compound evaporates and disperses at the time of sintering, thereby forming pores. Powders of aluminum fluoride, bismuth fluoride, zinc fluoride, lead fluoride and cadmium fluoride are described as examples of the compound added. It is stated that this compound is gradually transpired in the sintering step and completely transpired at a predetermined temperature. The pores formed after the transpiration of the compound shrink by being exposed to a high temperature of 1,500° C. and therefore, the resulting pores do not remain to effect a high capacitance. As a result, the produced capacitor is caused to have a very small capacitance of 10 to 16 μF and a very large tan δ of 7 to 9%.
JP-A-56-45015 discloses a method for producing an anode, where a substance having a melting point lower than a valve-acting metal powder such as tantalum and niobium is added to the powder, and the substance added is transpired by exposing it to a high temperature in a vacuum, thereby forming pores. Zinc and aluminum metals are described as examples of the substance added. In this method, the transpiration also takes place during sintering and therefore, due to shrinking caused by high temperature, the pores cannot remain to effect a high capacitance, as a result, the produced capacitor have a very small capacitance of 2.4 to 2.5 μF and a very large ESR of 140 mΩ.
JP-A-10-275746 discloses a method for producing a sintered body for capacitors, where vacancies are caused to remain by sintering a valve-acting metal powder having mixed therewith a granular binder of 50 μm or less in a vacuum at a high temperature of about 2,000° C. Polyvinyl alcohol, polyvinyl butyrate, polyvinyl acetate, polyethylene carbonate, methyl methacrylate, polyethylene-based resin, polyester-based resin and methacrylic resin are described as examples of the granular binder. These binders are organic polymers and decomposed and dissipated through a reaction such as depolymerization during sintering, which does not allow the pores to remain effective to achieve a high capacitance.
JP-A-11-181505 (U.S. Pat. No. 6,024,914) discloses a method for producing an anode sintered body for solid electrolytic capacitors, where an aggregated or granulated powder of a valve-acting metal such as tantalum, niobium and aluminum is shaped in combination with a solid organic material and the shaped article is sintered at a high temperature in a high vacuum. It is stated that a PVA (polyvinyl alcohol)-based or acrylic solid binder or camphor is preferred as the solid organic material. These solid organic materials are compounds of the same kind as the binder described in JP-A-10-275746, which are decomposed and dissipated through a reaction such as depolymerization during sintering and therefore, the pores cannot effect a high capacitance. As a result, although some improvement is seen in the ESR, the ESR of the sintered body is still as high as about 150 mΩ.
JP-A-9-74051 discloses a method for producing a sintered body for solid electrolytic capacitors, where a fiber comprising a methacrylic acid ester polymer, a polyethylene carbonate, a polypropylene carbonate, a polybutylene carbonate or the like after processing into a string shape is added and mixed to a valve-acting metal powder such as tantalum, titanium, niobium or aluminum and then the powder is shaped and sintered at a high temperature in a vacuum. These compounds are compounds of the same kind and have the same properties as the binder described in JP-A-10-275746 except for having a fiber shape, these compounds are decomposed and dissipated through a reaction such as depolymerization during sintering, and the pores cannot effect a high capacitance.
JP-A-6-252011 discloses a method for producing a porous sintered body for capacitors by shaping and sintering a valve-acting metal powder, where a thin sintered body is produced so as to shorten the vacancy path and a large number of the sintered bodies are stacked. In this method, more steps are required to stack a large number of thin sintered bodies for a capacitor unit than in a method where a single sintered body constitutes a capacitor, and this is economically disadvantageous, and at the same time, the capacitance efficiency is inferior, and a capacitor having a large capacitance cannot be produced.
JP-A-4-136102 (U.S. Pat. No. 5,082,491) discloses a technique where a liquid binder is mixed with a tantalum powder for electrolytic capacitors and the resulting granulated powder is graded to a particle size of 20 to 400 μm to obtain good flowability, then shaped and sintered. By grading the particle size, fine particles are removed and the void between granulated powder particles is once enlarged. However, this void becomes small due to compression at the shaping and a void almost the same as in the case of not performing the grading results. Therefore, the physical properties such as ESR cannot be improved.
JP-A-2001-345238 discloses a method for producing a porous sintered body, where a powder obtained by reducing potassium tantalum fluoride at 850° C. is ground by a bead mill to obtain a tantalum power having an average particle size of about 2 μm, the tantalum powder is mixed with a camphor emulsion, atomized and dried, the resulting granulated powder having a weak aggregating power is heated at 1,100° C. to produce a hard aggregated powder, the hard aggregated powder is mixed with magnesium chips and subjected to a deoxidation reaction at 800° C. to form vacancies of 1 to 20 μm, and the resulting tantalum or niobium aggregated powder is press-shaped to have a density of 4.5 to 5.0 g/cm3 and then sintered at 1,000 to 1,400° C. to obtain a porous sintered body having a pore peak in the ranges from 0.08 to 0.5 μm and from 1 to 20 μm, with 5 vol % or more of the entire vacancy volume having a vacancy size of 1 to 20 μm. In Examples, a sintered body where from 7 to 9 vol % of the entire vacancy volume has a vacancy size of 1 to 20 μm is disclosed, however, vacancies of 1 μm or less occupy 90 vol % or more of the entire vacancy volume and these are very small vacancies having a vacancy peak top in the range from 0.08 to 0.5 μm. Therefore, the sintered body cannot be sufficiently impregnated with the cathode material and for example, a sintered body of 99,000 to 101,000 μFV/g produced in this method is caused to have a very high ESR of 550 to 600 mΩ. Furthermore, the method involves steps of exposing the sintered body to a high temperature of 800° C. or more as many times as four and the more times the sintered body is exposed to heat, the more the CV may disadvantageously decreases.
International Publication WO 02/092864 discloses a method for producing a niobium sintered body having a pore diameter distribution where two peak tops are present, one in the range from 0.2 to 0.7 μm and the other in the range from 0.7 to 3 μm, and the latter peak top has a large relative intensity. It is stated that the niobium sintered body having these two peaks can be produced by adjusting the applied pressure at the shaping to a specific applied pressure value. In this method, when the applied pressure value is decreased, the adhesive strength between the lead wire as the electrode wire and the shaped article sometimes decreases to increase the LC. Furthermore, in the case of a large sintered body having a volume exceeding 25 mm3, the impregnation ratio of the cathode material does not reach 80% in some cases due to insufficient formation of pores and high capacitance and low ESR cannot be achieved at the same time.
JP-A-6-128604 discloses a method for producing a dense sintered member, where a metal working out to a matrix, such as tungsten, molybdenum, tantalum, niobium, chromium, cobalt, rhenium, iron, nickel and copper, is mixed with an oxide having an oxide-producing free energy lower than that of the matrix metal and having an oxygen amount lower than in the stoichiometric composition and then sintered. This method utilizes the activity of an oxygen-deficient oxide acting as a reducing agent. Aluminum and yttrium are described as examples of the oxygen-deficient oxide. In the sintered member produced by this method, the relative density is from 93 to 99% and almost no vacancy is present. Therefore, the sintered member cannot be impregnated with the cathode material and a capacitor having high capacitance and low ESR cannot be produced.
Under these circumstances, the present invention has been created and an object of the present invention is to provide a valve-acting metal sintered body for use as the anode of a solid electrolytic capacitor, mainly comprising valve-acting metal, a valve-acting metal compound and/or a valve-acting metal alloy, and a production method of the sintered body. The sintered body of the present invention, which can be sufficiently impregnated with cathode material and realizes a high capacitance with a low ESR and good tan δ property, enables production of a capacitor ensured with excellent long-term reliability such as humidity resistance and high-temperature loading. In particular, the sintered body is a high-capacitance sintered body having a CV value exceeding 40,000 μF·V/g, especially a large sintered body having a volume of 10 mm3 or more.
The method for producing a sintered body for solid electrolytic capacitors of the present invention, which comprises a step of compressing and shaping a valve-acting metal powder for capacitors to form a shaped article and a step of sintering the shaped article at a high temperature, is characterized in that the shaped article is formed from a granulated product comprising a mixture containing a pore-forming agent, an organic binder and a primary powder or secondary aggregated powder of valve-acting metal or a granulated powder thereof, and that the pore-forming agent is removed after sintering the shaped article. Alternatively, the shaped article can be formed by coating and printing dispersion liquid of the granulated product comprising the mixture. Fine pores can be obtained in the sintered body by conducting the sintering with a pore-forming agent which is unremovable at the sintering temperature and removing the pore-forming agent remaining in the sintered body after sintering is completed. The position, number and amount of pore peaks optimal for the cathode material used can be adjusted by controlling the kind, average particle size and amount added of the pore-forming agent, thereby enhancing impregnation degree of the cathode material.
According to the method of the present invention, the position, number and amount of peaks in the pore diameter distribution of the sintered body used as the anode can be controlled. Particularly, in a large sintered body having a volume of 10 mm3 or more and a porosity of 55 vol % or more, the volume of pores of 1 μm or more occupies 10 vol % or more of the entire vacancy volume. Therefore, a solid electrolytic capacitor, enhanced in impregnation of cathode material, having high capacitance, low ESR, good tan δ property and long-term reliability can be produced.